It has been known for some time that nitroaromatic compounds will bind to hypoxic cells in living matter. Hypoxic cells are those that are deficient in oxygen, yet remain viable. Hypoxic cells are most commonly found in tumors; however, it is understood that such cells may also exist in other living tissue, for example, heart tissue of a heart having undergone cardiac arrest or that has suffered the consequences of a stroke.
With specific reference to cancerous tissue, as tumors enlarge, the tumor tissue often outgrows its supply of oxygen and nutrients, because of an inadequate network of functioning blood vessels and capillaries. Although cells deprived of oxygen and nutrients ultimately die, at any given time a tumor may possess viable hypoxic cells; i.e., those cells which are deficient in oxygen, but are still functioning.
Radiation treatment is commonly used to destroy tumors. It has been determined that the rate of killing by radiation is 2.5 to three times higher for cancerous cells which contain oxygen than cancerous cells which are hypoxic. Hence hypoxic cells are more likely to survive radiation therapy and because they remain viable, eventually lead to the reappearance of the tumor. Chapman discusses this problem in "Current Concepts in Cancer", The New England Journal of Medicine, 304: 1429-1432, Dec. 27, 1979. There is at present, however, no practical and generally useful way of estimating the extent of hypoxia in a given tumor in a particular patient. On the other hand, if a measure of hypoxia were available, there are therapeutic interventions which can be designed to deal with the presence of hypoxic cells in the treatment of the tumors.
It has been known since about 1970 that certain nitroaromatic compounds will metabolically bind to hypoxic cells. A variety of techniques have been developed based on this discovery to determine the presence of hypoxic cells in living tissue. The clinically viable way of positively identifying and quantifying hypoxic tissue in tumors is by the use of radioactively labelled 2-nitroimidazoles. The reactive chemical is injected into tumor bearing animals or humans. The excess chemical is allowed to wash out and biopsy samples are investigated by scintillation, counting and autoradiography. This technique is disclosed in more detail by Urtasun et al, 1986, "A Novel Technique for Measuring Human Tissue pO.sub.2 at the Cellular Level", Br. J. Cancer, 54, 453. This approach can provide important information on the degree of tumor hypoxia, but is not generally acceptable because of the stringent requirements associated with handling radioactive tissues and body fluids. The delay required for autoradiographic analysis of the tissue sections is also a limitation with respect to the treatment plan. Nevertheless, the studies with radioactively labelled drug have established the usefulness and feasibility of the overall concept.
Agrawal, U.S. Pat. No. 4,282,232, discloses nitroimidazoles useful in radio-sensitizing hypoxic cells. The compounds are administered at acceptable levels to sensitize hypoxic cells such that, when treated with radiation, the now more sensitive hypoxic cells are killed. A further example of the use of nitroimidazoles in this manner is disclosed in U.S. Pat. No. 4,241,060.
The methodology, according to this invention, provides a technique for estimating the degree of hypoxia in tumors by existing medical laboratories which can be carried out in the same manner as other standard histochemical analyses of tumor tissue. Quick feedback is then provided by this invention so that adjustments in the treatment may be taken to deal with the hypoxic cells without any concern for radioactively labelled materials.